1. Field of the Invention
This invention relates to a port structure of a cylinder head, and more particularly a port structure in which a variation of a passage sectional area and a sectional shape of the port is reduced without increasing the size of the cylinder head.
2. Description of Background Art
An intake port shape of a cylinder head is normally set by assuring a back thickness of a valve guide fixing seat for its processing work and then connecting an opening part of the cylinder head with a line capable of assuring a minimum back thickness of the valve guide fixing seat. In this case, it is normally found that the center of the intake port approximately coincides with the center of the valve as seen from a valve stem side. Due to this fact, the back thickness for a machining of the valve guide fixing seat must be assured over an entire region of the port in its width direction. As a result, the upper side shape of the port is defined in reference to a machining position of the valve fixing seat (one example of the cylinder head with an intake port and an exhaust port formed therein is disclosed in Japanese Patent No. 2,628,138).
In general, a passage sectional area of the port is a location where the lower side of a seat machining part of the aforesaid valve guide fixing seat shows the minimum sectional area and in order to assure the necessary passage sectional area at this location, the size of the port in its width direction is adapted to be wide. However, the adaptation for a size only in the width direction requires that the size of the cylinder head must be increased, resulting in certain limitations. In addition, even if the size of the cylinder head is increased, an excessive flat port may not improve the intake and charging efficiency. On the other hand, when a sufficient amount of vertical height of each of the ports is assured while an intake and charging efficiency is being preferentially applied, the machining position for the valve guide fixing seat is necessarily required to be raised, resulting in that a total height of the engine is increased and the valves themselves are elongated and their weight also is increased. In particular, in the case of a certain type of engine in which its entire height is required to be shortened such as an engine that is vertically installed or mounted wherein the crank shaft is directed in a forward or rearward direction of the chassis, it may easily be restricted by the shape of each of the ports. In view of this fact, it is an object of the present invention to solve the problems described above.
In order to solve the problems described above, the cylinder head structure of an internal combustion engine of the present invention is constructed such that the cylinder head is provided wherein each intake valve and exhaust valve and each intake port and exhaust port and a valve seat machining part for use in fixing a valve guide of said valve is formed near said ports. The present invention includes at least one of the intake port or the exhaust port that is bent to any of right or left sides near said seat machining part and the center of each of the ports is offset from a back thickness part of the seat machining part.
Since the center of each of the ports is offset from the center of the valve stem as seen from the valve stem, the upper side position of each of the ports can be set to be higher than the seat machining part while assuring the minimum back thickness of the seat machining part, so that it is not necessary to adapt it only with the width of the port for assuring the passage sectional area at the port with the result that the passage sectional area can be sufficiently assured and the shape has a less amount in variation of the sectional area. Accordingly, it becomes possible to maintain an increased weight of the engine or a superior operation of each of the valves and at the same time if it is applied to the intake port, it is possible to improve an intake and filling efficiency.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.